Crappy Design

Not sure if you're kidding around here, but it shouldn't ever pick up atmospheric O2 concentration. The way an oximeter works is that it uses a very specific band of light that interacts differently with heme molecules (or hemoglobin protein, I can't fully recall) when they're saturated with something (usually oxygen) vs not saturated with something. That's what it's also called an "O2 saturation" or "O2 sat". You can calculate how much is saturated by how much light is absorbed, and that's what it does and why it won't read atmospheric concentration. It's important to note here that it only reads how much hemoglobin is bound to anything, including both Oxygen and Carbon Monoxide. So, you can asphyxiate from CO poisoning while having an SpO2 of 100%.